1. Field of the Invention
The present invention relates to an organic electroluminescent display, and more particularly, to an organic electroluminescent display with a pixel circuit for supplying a reverse bias voltage to an Organic Light-Emitting Diode (OLED) provided in a pixel.
2. Description of the Related Art
An organic electroluminescent display displays an image by supplying a data signal to a self-emissive OLED, and is classified as either a passive matrix or an active matrix organic electroluminescent display according to a driving method.
In a passive matrix organic electroluminescent display, anodes and cathodes of an image display region intersect in the form of a grid, and a pixel is formed in a region where the anode and the cathode intersect each other.
On the other hand, in an active matrix organic electroluminescent display, thin film transistors are disposed in respective pixels to control each pixel.
The biggest difference between the passive matrix organic electroluminescent display and the active matrix organic electroluminescent display is the emission time of the organic electroluminescent display. That is, the passive matrix organic electroluminescent display makes an organic emission layer emit light momentarily with a high brightness, while the active matrix organic electroluminescent display makes the organic emission layer emit light continuously with a low brightness.
In the passive matrix organic electroluminescent display, the momentary emission brightness must increase as the resolution increases. The high brightness deteriorates the organic electroluminescent display. On the contrary, in the active matrix organic electroluminescent display, the thin film transistor is used in driving the pixel, and the pixel emits light continuously in one frame, so that the active matrix organic electroluminescent display can be driven by a low current. Therefore, the active matrix organic electroluminescent display has advantages in that parasitic capacitance and power consumption are low compared to the passive matrix organic electroluminescent display.
However, the active matrix organic electroluminescent display has non-uniform brightness. In general, the active matrix organic electroluminescent display employs a Low-Temperature Polysilicon (LTPS) thin film transistor as an active device. The LTPS thin film transistor is crystallized by supplying a laser to amorphous silicon formed at a low temperature.
The characteristics of the thin film transistor vary depending on the crystallization. For example, the threshold voltage, etc. of the thin film transistor is not uniform for all pixels. Thus, the pixels display different brightness with regard to the same data signal, thereby allowing the whole image display region to have non-uniform brightness. Various attempts have been made to solve the non-uniform brightness problem.
The non-uniform brightness problem can be solved by compensating for the characteristics of a driving transistor. Methods of compensating for the characteristics of the driving transistor are broadly divided into two categories according to a driving method. That is, there is a voltage programming method and a current programming method.
In the voltage programming method, a voltage corresponding to the threshold voltage of the driving transistor is stored in a capacitor, and the threshold voltage of the driving transistor is compensated for by the stored voltage.
In the current programming method, a current is supplied as the data signal, and a voltage difference between a source and a gate of the driving transistor corresponding to the supplied current is stored in the capacitor. Then, the driving transistor is connected to a power supply, so that a driving current corresponding to the supplied current flows in the driving transistor. Thus, the driving current supplied to the organic emission layer is corresponding to the current supplied as the data signal, regardless of the different characteristics of the driving transistors. Therefore, the brightness problem is reduced.
However, the foregoing methods for improving the brightness problem are based on the assumption that the organic electroluminescent display has a normal organic emission layer. If the organic emission layer has defects, such as a pinhole formed in a fabrication process, the organic electroluminescent display cannot emit light normally even though a difference in characteristics of the driving transistors is compensated for.
In the case of the organic electroluminescent display having defects like as a mura, the defects are generally detected by examining a displayed image of the organic electroluminescent display while the organic electroluminescent display is operated normally. However, this method cannot check for progressive defects in the organic electroluminescent display, and must drive a plurality of transistors corresponding to the pixels.
Accordingly, there is a demand for an organic electroluminescent display whose pixels can be electrically checked for defects without having to display an image.